Conventionally, a spark plug is used in an internal combustion engine such as a gasoline engine and a gas engine. For example, Japanese Patent Application Publication No. 2015-130302 (corresponding to U.S. Patent Application Publication No. 2015/194793) discloses a spark plug including an auxiliary combustion chamber. In this spark plug, the auxiliary combustion chamber is formed in a cap fixed to a front end portion of a metal shell. The cap includes a hole connecting the auxiliary combustion space and the outside. The fuel gas is introduced through the hole of the cap into the auxiliary combustion chamber. Moreover, a center electrode and a ground electrode are disposed within the auxiliary combustion space. Spark generated in a gap between the center electrode and the ground electrode ignites the fuel gas introduced into the auxiliary combustion space. Then, the flame is discharged through the hole of the cap to the outside, that is, to the combustion chamber of the internal combustion engine, so that the fuel gas within the combustion chamber is combusted.
However, the above-described art is not sufficiently designed for the pressure loss and the heat loss which are generated within the auxiliary combustion space. Accordingly, the pressure loss and the heat loss which are generated within the auxiliary combustion space are excessively increased, so that it may not be possible to obtain the sufficient ignition performance (for example, stability of the combustion).
An advantage of the present invention is a spark plug which includes an auxiliary combustion space, and which is designed to solve the above-mentioned problems, and to improve the ignition performance.